The present invention relates to a sensitizing activator for chemical plating. Especially, the present invention relates to a sensitizing activator for use in chemical plating of through holes of a multi-layer printed circuit board. More particularly, the present invention relates to an improved sensitizing activator having an activity of uniformly and efficiently depositing a substance capable of catalytically depositing a plating metal from a plating solution, such as colloidal palladium, on the surface of a material to be plated in advance.
It is known that in conventional plating the operation of depositing a catalytic substance such as colloidal palladium on the surface of a resin should be performed before chemical plating. In order to reduce the manufacturing cost of a chemically plated product, how to deposit this colloidal palladium uniformly and efficiently is one of the important problems. The inventor previously found that by using an activating treating agent formed by adding vanillin and an acidic dye to a hydrochloric acid solution of palladium chloride, the amount of palladium used can be reduced and a product excellent in the plating quality over a conventional product can be obtained (see, for example, Japanese Patent Application No. 136473/1974). The inventor furthered his research and found that when a hydrochloric acid solution of palladium chloride and tin chloride is used instead of the hydrochloric acid solution containing palladium chloride alone as taught in the invention of Japanese Patent Application No. 136473/1974, colloidal palladium can be efficiently deposited and a product improved in the plating quality can be obtained by adding vanillin alone without using any acidic dye, as disclosed in Japanese Patent Publication No. 34220/1980.
The sensitizing activator proposed in the Japanese Patent Publication No. 34220/1980 (hereinafter referred to as "conventional technique") is effective in the sensitizing step in chemical plating of excellent quality plated through holes of a single layer printed circuit board. However, when this sensitizing activator is used for plating of through holes of a multi-layer printed circuit board, hydrochloric acid contained in th sensitizing activator causes corrosion of the black oxide that is normally formed on an inner copper foil of a copper-clad resin laminate board in the inner wall portions of the through holes of the copper-clad board, and the sensitizing activator permeates between the copper-clad resin laminate layers and is left therein and, since hydrochloric acid contained in the sensitizing activator has an electrical conductivity, the insulation between adjacent through holes becomes insufficient or the bonding between the copper-clad resin laminate layers is weakened to cause delamination.
Accordingly, in the case where the conventional sensitizing activator is used in the step of plating through holes of a multi-layer printed circuit board, the conditions such as dipping time, the concentration and the temperature are restricted and the process control is difficult. Namely, it is difficult to shorten the dipping time or reduce the hydrochloric acid concentration so that the black oxide of the inner copper foil of the resin laminate board is hardly corroded, and the sensitizing treatment is insufficient.
Therefore, there is a need for a sensitizing activator having an excellent performance, in which the above-mentioned defects are overcome.